In most internal combustion engines, the air intake valve remains open during an initial portion of the compression stroke or will open prior to the completion of the exhaust stroke of the engine. These conditions, when they occur, may produce a momentary back air flow through the engine's air intake manifold. This momentary lack of air flow is most pronounced for four cylinder engines within a defined engine speed range.
Thermal mass air flow sensors of the type taught in U.S. Pat. No. 4,974,445, U.S. Pat. No. 5,231,871 and U.S. Pat. No. 4,911,009 have a high response rate and do not require compensation for the pressure of the air flow being detected. However, these thermal mass air flow sensors are not capable of discriminating the direction of mass air flow in the engines air intake manifold and thus will produce an output in response to the back air flow as well as the forward air flow. As a result, the mass air flow sensor will detect the back air flow and produce a faulty mass air flow signal which causes an associated electronic fuel control computer to deliver an excessive amount of fuel to the engine when operating within the defined engine speed range.
In FIG. 1, curve 4 shows an approximation of an actual mass air flow required by a four cycle, four cylinder internal combustion engine as a function of engine speed given in revolutions per minute (RPM).
The mass air flow rate in FIG. 1 is given as a typical output voltage of the mass air flow sensor. Curve 6 depicts the mass air flow as measured by a mass air flow sensor mounted in the intake manifold of the engine. As is apparent from FIG. 1, the output (voltage) signal generated by the mass air flow sensor in the engine speed range between 1000 and 2500 RPM is significantly distorted by the back air flow produced by the engine between the opening of the engine's intake valve and the completion of the exhaust stroke of the associated piston. Unfortunately, the speed range at which the output signal generated by the mass air flow sensor is distorted is within the speed range of the engine for nominal vehicle operations as may be encountered in city driving.
This problem has been addressed by Oyama et al. in U.S. Pat. No. 4,478,075. Oyama teaches the application of the back air flow to reduce the value of the forward air flow so that the resultant measure air flow is substantially equal to the air flow required by the engine.